This invention relates to a pneumatic spring.
The principle of such springs has been known of for a long time: a cylindrical enclosure is divided into two chambers by a piston integral with a rod which projects from the enclosure through an impervious seal. The two chambers are joined together by a passage which includes a constriction. When the enclosure is filled with a gas under pressure, the rod is subjected to a force which causes it to come out of the enclosure, and which is proportional to the relative pressure in the enclosure and to the transverse section of the rod, and the movement of the rod is carried out at a speed which is governed by the flow of fluid through the constriction connecting the two chambers.
If the relative pressure of the fluid is zero, that is, if the fluid is at generally atmospheric pressure, then the device functions purely as a damping device.
If, on the other hand, the pressure of the fluid is high and if the constriction allows a substantial flow, then the device functions principally as a spring.
The higher the pressure in the enclosure, the more likely it is that damage resulting in a severe drop in pressure can have unfortunate consequences on an installation to which the spring-damping device is connected. In fact, due to the damage, the pressure drops rapidly in the chamber where the damage has been caused, while the action of the constriction only permits a slow drop in pressure in the other chamber. A phenomenon similar to the sudden, and undesired, slackening of a spring, with rapid and uncontrolled movement of the rod towards one of its extreme positions, is produced.
This can be seen particularly in the case of a device such as is described in French Pat. No. 1,336,987 relating to a "pneumatic spring" containing a gas at a pressure which can reach 150 bars.
Devices are already known in which the means for restraining the movement of a piston of a pneumatic spring are controlled by the pressure in a variable volume chamber. For example, French Pat. No. 2,444,854 describes a variable volume chamber which, when in its inactive state, is isolated from the enclosure of the pneumatic spring and contains a certain quantity of gas. When there is a fall in the pressure in the enclosure, a valve connects the variable volume chamber with the enclosure, which sets off the braking system. This system has the disadvantage, if it is sensitive, of being able to be set into action by a simple momentary difference in temperature between the chamber and the enclosure, without easily being returned to its previous state.
U.S. Pat. No. 2,928,507 provides for a braking action of the piston by the action of a variable volume chamber, but only in the case of over-pressure in the enclosure, which is contrary to the objective aimed at here. In this patent also, the enclosure is not connected to the chamber under normal operating conditions, which presents the same type of disadvantage as that described above.
The aim of the present invention is therefore to provide a device which is sensitive, that is, which reacts rapidly to a drop in pressure in the enclosure, which is not subject to being set in motion at inopportune moments and which, furthermore, is simple, strong and economical.